1. Field of the Invention
This invention relates to an acoustic optical light modulator used for modulating a laser beam in a system utilizing the laser beam, such as a laser printer, a facsimile, a video disk or a laser display and in particular to an acoustic optical light modulator in which ultrasonic waves are utilized for modulating the intensity of laser beam.
In such a light modulator, so called a ultrasonic wave light modulator and an ultrasonic wave media having a constant refractive index are used to propagate ultrasonic waves of a periodic density in a certain direction in the media and a laser beam incident in the ultrasonic wave media at an appropriate angle to the direction of the ultrasonic waves is modulated to be a diffractive beam reflected on a plane of the ultrasonic waves. The principle of modulating a laser beam in said ultrasonic wave media is varying the density of the ultrasonic waves propagated in the media by regulating the output of the ultrasonic waves thereby modulating the intensity of the diffractive beam reflected on the plane of the ultrasonic waves.
2. Description of Prior Art
An example of a conventional acoustic optical light modulator is explained with reference to FIG. 1. In FIG. 1, reference numeral 1 designates a hexahedral ultrasonic wave media having a bottom 1a and two sides 1b and 1c perpendicular to the bottom. A ultrasonic wave generator 2 provided with two electrodes 2a and 2b is attached to the bottom 1a of the media 1. Connected to the electrodes 2a and 2b is a high frequency oscillator 3 controlled by a video signal. If a frequency modulated voltage from the high frequency oscillator 3 controlled by a video signal is applied to the two electrodes 2a and 2b of the ultrasonic wave generator 2, the generator 2 propagates ultrasonic waves 4 in the ultrasonic wave media 1 in the direction of the electric field in proportion to the voltage applied thereto. The ultrasonic waves 4 propagated in the media 1 are the periodic compressional waves and the intensity of the output of the ultrasonic waves 4 generated by the ultrasonic wave generator 2 varies in compliance with the amplitude of the voltage applied thereto. However, since the frequency of the ultrasonic waves is kept constant, the periodicity of the compressional waves is also kept constant to form equidistant light reflective planes 4a of close ultrasonic wave portions. Thus, if a laser beam 6 radiated from a laser source 5 is incident in the ultrasonic wave media 1 through one side 1b of the media, the incident laser beam 6 is reflected on one of the above-described light reflective planes 4a formed in the media and a reflected diffractive beam 6 a leaves out of the media 1 through the other side 1c of the media. In other words, if the laser beam 6 is incident upon the light reflective plane 4a in the ultrasonic wave media under the condition satisfying the Bragg's Law set forth below, the reflected laser beam becomes the diffractive beam 6a, EQU 2.lambda..sub.s Sin .theta.=.lambda. (1)
wherein .lambda..sub.s is the wavelength of the ultrasonic wave, .lambda. is the wavelength of the laser beam and .theta. is an incident angle at which the laser beam is incident upon the plane of the ultrasonic waves.
Then the intensity of the diffractive beam 6a can be varied if the density of the light reflective plane 4a of the ultrasonic waves is varied by controlling the output from the ultrasonic wave generator 2, so the laser beam 6 can be intensity modulated according to the following formula, EQU I=I.sub.0 Sin.sup.2 [.lambda.(p.multidot.L).sup.1/2 /.lambda.](2)
wherein I is the intensity of the diffractive beam, I.sub.0 is the intensity of the incident laser beam, P is the intensity of output of ultrasonic wave, L is the width of the ultrasonic wave media and .lambda. is the wavelength of the laser beam used.
In the conventional light modulator described, as the both sides 1b and 1c of the ultrasonic wave media 1 are perpendicular to the bottom 1a, the laser source 5 should be mounted slantwise at a certain angle with respect to the side 1b of the media 1 in order that the laser beam 6 is incident upon the plane 4 of the ultrasonic waves in the media under the condition satisfying the Bragg's Law, that is at an angle .theta. to the plane 4, accompanying the inconveniences in designing and manufacturing the system. Moreover, since the light reflective plane 4a in the ultrasonic wave media 1 is formed of close ultrasonic waves, the plane cannot constitute a perfect light reflective surface and especially due to reduction in the output of the ultrasonic waves caused by diffusion of the ultrasonic waves, only less than 70% of the laser beam 6 is modulated to be the diffractive beam 6a. A non-diffractive beam 6b transmits the equidistant light reflective plane 4a and a reflective beam 6d is reflected at the side 1c. The remainder is not modulated to pass through the media 1 without being used, thus not only causing great loss in the laser beam but also requiring an additional light absorbing body 7 mounted outside the media. Furthermore, loss of light due to the reflective beam 6c on the surface of the side 1b is unavoidable and so is the instability of light modulation caused by the effect of heat generated in the media 1 in case that a high power laser beam is used such as Ar.sup.+ or Kr.sup.+ laser beam.